Subtropical high

Also called: Horse latitudes · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

A subtropical high is one of two belts of high air pressure at about 30°–35° N and S [3]. Air that rose at the equator piles up here, sinks and warms, so the skies stay clear and dry. These belts explain why the world's great hot deserts, including the Thar in India, lie near 30° N/S. They are also the classic example of a dynamic pressure belt, which UPSC tests often.

Explanation

How the belt forms (the Hadley cell)

• Air rises at the equator.
• Strong sun heats the air, so it expands and rises.

• High in the sky, this air flows towards the poles [2].

• The air turns and slows down.

• On its way it gives off heat and cools.
• The Coriolis force (the turning effect of Earth's rotation) bends moving air to the right in the Northern Hemisphere and to the left in the Southern.

• The air ends up moving almost east–west, so it cannot travel further towards the pole.

• The air piles up and sinks.

• The air gathers near 30°. This extra weight of air sinks and creates high pressure at the surface.

• The loop closes.

• Part of the sinking air flows back to the equator near the ground as the trade winds [2].
• This loop is the Hadley cell: air rises at the equator, moves towards the poles high up, sinks near 30° and returns along the ground. It is named after George Hadley (1735) [2].

Why it is "dynamic", not "thermal"

• Thermal origin: temperature causes the pressure directly. Heat gives a low (the equator). Cold gives a high (the poles).
• Dynamic origin: air movement and Earth's rotation cause the pressure, not local temperature.
• The 30° belt is hot but still high-pressure. This is the proof that it is dynamic.
• On sea-level maps (pressure readings converted to sea level before drawing isobars, lines that join places with equal pressure), it is one of seven pressure belts. It sits between the equatorial low and the subpolar low.

Why it brings no rain

• Subsidence (air sinking slowly over a wide area) squeezes the air and warms it. Warm air can hold its water vapour, so clouds do not form.
• Temperature inversion (a layer where air gets warmer with height instead of colder) sits on top like a lid. It stops moist air from rising.
• Result: light winds, clear skies [3], dry belts and hot deserts.
• The name horse latitudes comes from these calm winds. The usual story is that sailing ships stuck here threw horses overboard to save water.

Cells and seasonal shifting

• Belts break into cells. Land heats and cools faster than sea. Because the Northern Hemisphere has so much land, its continuous belt splits into separate oceanic highs called centres of action.
• Azores–Bermuda high in the North Atlantic.
• Hawaiian (North Pacific) high in the north-east Pacific.

• Both last all year and grow stronger in summer.

• The belt moves with the Sun.

• It shifts about 5–10° north in the northern summer (June) and south in the northern winter (December).
• This is because the Sun is overhead at the Tropic of Cancer (23.5° N) around 21 June and at the Tropic of Capricorn (23.5° S) around 22 December.

Where it occurs

• Belts: about 30°–35° N and S [3] around the globe.
• Oceanic cells: the Azores–Bermuda high (North Atlantic) and the Hawaiian high (north-east Pacific). In the Southern Hemisphere, which is mostly ocean, the belts are more continuous.
• Hot deserts under the belt (20–30°, on the west sides of continents):

Hemisphere	Desert	Location
N	Sahara	North Africa
N	Arabian	Saudi Arabia, West Asia
N	Thar	Rajasthan (India) and Sindh (Pakistan)
N	Sonoran / Mojave	American South-west, USA–Mexico [2]
S	Kalahari (with Namib)	Botswana, Namibia
S	Atacama	Chile, west coast of South America
S	Great Australian deserts	Western and central Australia

• Why these deserts are on west sides:
• The eastern edge of each oceanic high lies next to a continent's west coast. Air sinks most strongly here.

• Cold ocean currents along these coasts make the air even drier: the Canary (Sahara), Benguela (Namib–Kalahari), Peru or Humboldt (Atacama) and West Australian currents.

• Mediterranean lands (about 30–45°, west sides of continents) fall under the shifting high in summer: the Mediterranean basin, California, central Chile, the Cape region of South Africa and south-west Australia.

Don't confuse with

• Doldrums (equatorial low): a calm belt too, but it has low pressure, rising air and heavy rain at 0–5°. The horse latitudes have high pressure, sinking air and dry skies.
• Subpolar low (~60° N/S): also dynamic, but it is a low. Warm westerlies meet cold polar easterlies there and warm air is forced up, which brings stormy weather.
• Siberian high: a thermal high over land, formed in winter by very cold air over northern Asia. It is not a dynamic, year-round oceanic high.
• Polar high (90° N/S): a thermal high caused by very cold, dense air sinking over ice. The subtropical high is a hot belt with high pressure.

Prelims Hooks

• Horse latitudes = subtropical high belts at about 30°–35° N/S [3]. Doldrums = the equatorial low.
• Classification trap: the subtropical high and subpolar low are dynamic. The equatorial low and polar highs are thermal. "Which of these is NOT thermal?" → the subtropical high.
• Sinking air at 30° is the downward arm of the Hadley cell. Its return flow to the equator is the trade winds [2].
• Desert–current pairs: Atacama–Peru (Humboldt), Namib/Kalahari–Benguela, Sahara–Canary, Australian deserts–West Australian current.
• Azores–Bermuda high → North Atlantic. Hawaiian high → North Pacific. Both are subtropical highs that last all year and are stronger in summer.
• Trap: hot deserts lie on the west sides of continents under the subtropical highs, not on east coasts. India's Thar is one of them.

Mains Points

• Pressure belt → climate zone: sinking air, subsidence warming and the temperature inversion block rain at 30° N/S. This explains the belt of hot deserts across the American South-west, northern Africa and Australia [2], and the dry Thar in western Rajasthan. It is a ready framework for GS-I answers on how the world's climates are distributed.
• Pressure depends on more than temperature: the 30° belt is hot yet high-pressure. This shows that Earth's rotation and the three-cell circulation (Hadley, Ferrel and polar cells [2]) control pressure, not heat alone. Use it to explain dynamic versus thermal origin.
• Shifting high → seasonal climates:
• In summer the high moves north over Mediterranean lands, so sinking air gives a dry summer.
• In winter the belts move south, so the westerlies and their depressions bring winter rain.
• The same seasonal movement of belts with the Sun drives the reversal of the Indian monsoon.

Related concepts

• Isobar
• Pressure belts
• Subpolar low
• Polar high
• Shifting of pressure belts

Read more

• Horizontal distribution: isobars and the world's pressure belts